First, update to the latest version of Java here. Run the latest version of the sim directly from http://phet.colorado.edu by clicking the 'Play' button on the sim page. If you are using a Mac please see here for detailed instructions.

PhET simulations are written in three different programming languages: Java, Flash, and HTML5. These are each handled by a computer differently, and some may be compatible with your device while others not. See Windows, Mac, etc. for more answers.

Once you have that successfully installed Java, click 'Play' to download a .jnlp file. If you receive any messages about the file download, click the 'Keep' button. Navigate to the download location and open the .jnlp file. If you encounter any problems opening the .jnlp see this entry.

Several internet browsers, including Google Chrome and Safari 10, will begin blocking Flash by default by the end of 2016. Flash will still be bundled with Chrome, but users will be prompted to accept the page. Once a page has been accepted, all pages that display Flash content at that domain (phet.colorado.edu) will be allowed.